Passing a string from a structure to an array of strings - C

Question

typedef struct mensagem
{
    int sender ;
    int receiver ;
    char *text ;
} *Item ;

typedef struct node
{
    Item item ;
    struct node *next ;
} *link ;

typedef struct queue
{
    link head, tail ;
    int size ;
} *Queue ;

     void listsorted(Queue list)
    {
        Queue temp = list ;
        int s=temp->size ;
        char *sorted ;
        int i=0;

        sorted = (char*) malloc(sizeof(char));

        while ( i < s )
        {
            strcpy( sorted[i] , list->head->item->text ) ;
            list->head = list->head->next ;
            i++ ;
        }

        i=0;

        for ( i=0 ; i<s ; i++ )
        {
            printf("%s\n", sorted[i] ) ;
        }
    }

I want to sort alphabeticaly a queue, so I thought to copy the strings to an array and qsort that array. But i'm not even managing to make the array with the strings. I get an error on the strcpy . What am I doing wrong?

what error exactly are you getting? Is it at compile-time or at run-time? — Dan O
– Dan O, Commented May 12, 2014 at 14:31
I think it should be char **sorted; cause you want to have an array of strings. — mic4ael
– mic4ael, Commented May 12, 2014 at 14:35
That solves it in the compiler, but i get a segmentation fault when i enter the while — TJacob
– TJacob, Commented May 12, 2014 at 14:38

pah · Accepted Answer · 2014-05-12 14:35:32Z

1

You're not malloc()'ing both dimensions of the array. Take a look at the following code (untested):

 void listsorted(Queue list)
{
    Queue temp = list ;
    int s=temp->size ;
    char **sorted ; /* Changed here */
    int i=0;

    sorted = malloc((s + 1) * sizeof(char *)); /* Changed here */

    while ( i < s )
    {
        sorted[i] = malloc(strlen(list->head->item->text) + 1); /* Changed here */
        strcpy( sorted[i] , list->head->item->text ) ;
        list->head = list->head->next ;
        i++ ;
    }
    sorted[i] = NULL; /* Changed here. NULL terminate array */

    i=0;

    for ( i=0 ; i<s ; i++ )
    {
        printf("%s\n", sorted[i] ) ;
    }
}

answered May 12, 2014 at 14:35

pah

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1 Comment

TJacob Over a year ago

It compiles, but it is only printing the first string. EDIT: Nevermind, that was due to a problem in another part of the code, so it worked! Thanks.

Dan O · Accepted Answer · 2014-05-12 14:36:37Z

1

sorted[i] is of type char, not of type char*. That's because you're only allocating a one-dimensional array (i.e., a string) and not a two-dimensional array (i.e., an array of strings). Further, you're mallocing 1 byte then attempting to copy the contents of list->head->item->text into it. This will be bad news even if you get this code to compile.

Make sure you always allocate the correct amount of memory, and make sure you always use strncpy instead of strcpy.

answered May 12, 2014 at 14:36

Dan O

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